1. Field of the Invention
The invention relates to a semiconductor device having a metal gate and manufacturing method thereof, and more particularly, to a semiconductor device having a metal gate and manufacturing method applied with a gate last process.
2. Description of the Prior Art
With a trend toward scaling down the size of the semiconductor device, work function metals are used to replace the conventional polysilicon gate to be the control electrode that competent to the high-K gate dielectric layer. The conventional dual metal gate methods are categorized into the gate first process and the gate last process. Among the two main approaches, the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-K gate dielectric layer and the metal gate, and thus gradually replaces the gate first process.
In a conventional gate last process, a dummy gate or a replacement gate is formed on a substrate and followed by steps of forming a conventional metal-oxide semiconductor (MOS) transistor device and forming an inter-layer dielectric (ILD) layer. Subsequently, the dummy/replacement gate is removed to form a gate trench. And the gate trench is filled with metal layers required for different conductivity types. As the line width of the transistor device keeps shrinking, the aspect ratio of the gate trench becomes more and more critical when filling the gate trench with the metal layers. Briefly speaking, when the line width of the transistor device is reduced, the opening width of the gate trench is consequently reduced, and thus it is difficult to fill the gate trench with the metals, it even causes a seam left in the metal gate after forming the metal layers in the gate trench. Eventually, the electrical performance of the transistor device having the metal gate is deteriorated.
It is found that the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-K gate dielectric layer and the metal gate. However, the gate last process still faces integrity requirements for the complicated processes and reliability requirement for the gate trench filling.